PROJECT SUMMARY Staphylococcus aureus (S. aureus) is a Gram-positive bacterial pathogen capable of causing a wide range of disease in humans and animals alike. Despite the advent of antibiotic therapy, morbidity and mortality due to S. aureus infection remains a major health concern worldwide. Extensive efforts for over a century to produce a S. aureus vaccine for humans have universally failed. Yet, exactly how this pathogen undermines an effective immune response is not clear. Preliminary findings within this research proposal demonstrate significant differences in the association of Staphylococcus epidermidis with B cells in human blood compared to S. aureus. Further, a saeR/S mutant strain of S. aureus defective in expressing a number of extracellular virulence genes was able to associate with human B cells in a manner identical to that of S. epidermidis. These findings are significant in that the complement-mediated association of antigen with both antigen- specific and antigen-nonspecific B cells is essential for mounting an effective antibody response in vivo. Thus, the failure of S. aureus to associate with B cells in human blood might account for the inability of standard vaccination strategies to provide protective immunity against this pathogen. The Specific Aims outlined within this research proposal are designed to further explore the association of staphylococci with B cells. Specific Aim 1 will address the importance of complement and platelets for the association of B cells with staphylococci by using antibodies specific to complement receptors, complement depleted serum, and soluble complement receptors in flow cytometry and immunofluorescence microscopy assays. Specific Aim 2 of this proposal will identify SaeR/S-regulated factors expressed by S. aureus that prevent the association of this pathogen with B cells. Results from the proposed experiments will improve our understanding of how S. aureus influences the immune response in humans, potentially providing insight into the failure of previous vaccination approaches as well as a solid foundation towards developing novel vaccination strategies against this pathogen.